The present invention relates to a method for drying ceramic articles via a microwave dryer, and in particular to methods for drying ceramic honeycomb structures via a microwave dryer that promotes uniform drying of the honeycomb structures, thereby relieving or eliminating heat-induced structural degradation of the structures.
Ceramic honeycomb structures having transverse cross-sectional cellular densities of approximately one-tenth to 100 or more cells or channels per square centimeter of honeycomb cross-section have several uses, including use as particulate filter bodies, catalyst substrates, and stationary heat exchangers. Filter applications generally require that selected cells of the structure be sealed or plugged at one or both of the respective ends thereof in a manner such that wall-flow filtration, i.e., the filtering of fluids traversing the structure by directing at least some of those fluids through porous channel walls thereof, is effected.
Ceramic honeycomb manufacture involves several known steps. In general, the honeycomb shapes are first formed, e.g., by extrusion, from water-containing plasticized mixtures of ceramic raw materials. The formed honeycombs are next dried to solidify the desired honeycomb structure, and are finally fired to sinter or reaction-sinter the ceramic raw materials into strong unitary ceramic articles.
Referring to the appended drawings, the reference numeral 8 (FIG. 1) generally designates a ceramic article of a type that is well known for applications such as catalyst substrates and diesel exhaust particulate filters. The base structure in both cases is a ceramic honeycomb 10 comprising a matrix of intersecting, thin, porous cell walls 14 surrounded by an outer wall 15. In the illustrated example structure 10 is provided in a circular cross-sectional configuration including a first end 13, a second end 16 and a middle portion 17. The walls 14 extend across and between a first end face 18 and an opposing second end face 20, and form a large number of adjoining hollow passages or channels 22 which extend between and are open at the end faces 18, 20 of the structure 10.
To form a filter from structure 10 (FIGS. 2 and 3), one end of each of the cells 22 is sealed, a first subset 24 of the cells 22 being sealed at the first end face 18, and a second subset 26 of the cells 22 being sealed at the second end face 20 of the substrate 10. Either of the end faces 18, 20 may be used as the inlet face of the resulting filter. The structure 10 with seals is then fired to form the filter.
In operation, contaminated fluid is brought under pressure to an inlet face and enters the filter via those cells which have an open end at the inlet face. Because the cells are sealed at the opposite ends, i.e., the outlet face of the body, the contaminated fluid is forced through the thin porous walls 14 into adjoining cells which are sealed at the inlet face and open at the outlet face. The solid particulate contaminant in the fluid, which is too large to pass through the pore structure of the walls, is left behind and the cleansed fluid exits the filter through the outlet cells and is ready for use.
Some previous methods used for drying ceramic honeycomb structures have led to decreased structural strength due to heat-induced structural degradation. Structural strength requirements are particularly demanding for ceramic catalyst substrates and filters to be used in the mechanically harsh environment of motor vehicle exhaust emissions control systems. Nevertheless, for the mass production of such filters and substrates it is highly desirable to be able to dry the ceramic substrates rapidly and as inexpensively as possible, while maintaining structural integrity and strength.
Various drying techniques have been utilized for ceramic honeycomb manufacture in the past, including conduction heating, convection heating, and RF heating. Microwave heating has been used to achieve higher volumetric heating uniformity than conduction and/or convection heating can provide alone, while at the same time offering low operating costs and reduced processing times. However, some ceramic materials useful for constructing ceramic substrates and filters, particularly including batches for the manufacture of cordierite, mullite, aluminum titanate, and similar ceramics that include a graphite additive to increase honeycomb porosity, are more difficult to dry via microwave drying. Also problematic from a drying standpoint are honeycombs directly incorporating materials such as transition metal oxide catalysts, where the catalysts include constituents that are semiconductive or very lossy at the desired microwave drying frequency.
These drying difficulties are attributed to the inability of microwave radiation to properly penetrate into and effect uniform heating within the interior portions of such materials, due to reduced microwave permeability occasioned by the presence of graphite or other lossy materials within the ceramic batch mixtures. The consequence is that the drying of such honeycombs using microwave radiation can lead to unacceptable localized heating, which in turn leads to unstable processing, poor select rates, and lower quality ware. For example, the drying of an aluminum titanate substrate with a 30% graphite additive has produced unwanted edge heating that results in cracks and/or contour problems in the associated filter.
One possible solution to this drying problem is simply to remove damaged edge portions from the dried honeycomb parts. This solution is obviously inefficient and creates a significant amount of waste. Other solutions include changing the composition of the ceramic batch mixtures to reduce the amount of graphite or other lossy materials therein, or using multiple drying steps, or using a combination of drying methods, for example, microwave plus hot air drying, to achieve drying without structural damage. However, each of these alternatives requires accepting unwanted compromises, such as lower quality end products and/or increases in manufacturing costs.
A method for drying ceramic substrates that reduces unwanted nonuniform drying characteristics within the ceramic substrates, thereby reducing unwanted heat-induced stress cracking and structural degradation of the substrates, while simultaneously decreasing associated cycle times, and associated operating costs, is therefore desired.